In our U.S. Pat. No. 5,392,119 issued Feb. 21, 1995 entitled Plate Correction of Imaging Systems, we disclosed techniques for the correction of extant imaging systems. It is useful to repeat the Summary of the Invention from our prior disclosure, which is now U.S. Pat. No. 5,392,119. Because of the relevance of this disclosure, we incorporate the specification herein by reference.
An object is projected through the lens system to be corrected to the image plane where the position of the diffraction limited ideal image is readily ascertainable. At least one primary image defect is measured. These aberrations can include but are not limited to distortion, curvature of field, spherical aberration, coma, and astigmatism. Distortion, curvature of field and astigmatism are commonly measured by imaging simple known patterns through the system and analyzing their images. Interferometry can be used to measure the aberrations of the system. Based on non-interferometric measurements, the configuration of an apparent object can be simply computed, based on the optical systems parameters such as the magnification. The apparent object is an imaginary location of the object points which would cause the image of the apparent object through the real system to register to the ideal image. One or more plates and their mounts may be required to achieve the desired optical system performance improvements. The first corrector plate is optimized to produce beam deflection. This deflection is chosen so that rays from the object are given optimal displacement to intersect the desired ray trace path (i.e., the rays from the apparent object) at the second corrector plate to correct for the measured aberrations. The second corrector plate is designed so that rays at the second corrector plate, having been previously displaced to the desired ray trace path, are given the required direction to image at the ideal image. In the analysis offered, provision is made to insert inside the optical system a phase conjugate plate for the correction of (fixed) position independent aberrations, such as spherical abberation. A matrix of test points is utilized to locally measure and correct, on a stepper to stepper basis, lens systems through customized corrector plates individually tailored to each lens system. There results a lens system correctly customized which assists in approaching the diffraction limit of the optical system. Both the process and the product of the process is included in the disclosure.
It will be understood that this patent application particularly applies to optical trains found in steppers. As images in such steppers have been required to provide increasingly improved resolution to ever smaller circuits, the use of the techniques here disclosed enables extant steppers to be corrected and used for imaging beyond the design tolerances of their original fabrication. Further, it will be understood that the disclosed process and product of the process can be utilized for the initial lens train fabrication. In this latter case, production tolerance requirements of the original optical elements can be relaxed with reliance on the corrective process and product of the process herein disclosed.
This corrective process and product of the process has been particularized to that "ray trace" path that an idealized optical system would project, if that optical system is an optimum optical system. In the following disclosure, the particular path that the individual rays trace is not important. Instead, a more generalized correction scheme is set forth.